trulation is not essential for this locus. A study of 
additional chromosomal rearrangements is underway. 
Analysis of many adjacent nuclei in embryos has 
revealed that homologously paired nuclei (early in 
development) or residual unpaired nuclei (late in 
development) occur in patches, possibly indicating 
a cell lineage relationship. To analyze these features 
further, the group continues to develop methodol- 
ogy (in collaboration with Dr. Zvi Kam of the Weiz- 
mann Institute) for automatically locating and 
quantitating in situ such patches in thousands of 
nuclei on whole surfaces of embryos. 
Homologous chromosome pairing during meiosis 
was also investigated, using genetically and mo- 
lecularly characterized plant maize. A three- and 
four-dimensional analysis of leptotene-diakinesis 
chromosome structure is in progress. This is a col- 
laborative study with Dr. Zack Cande and his group 
at the University of California, Berkeley. 
Chromosomal Substructure Determination 
Dr. Sedat's laboratory continues to investigate the 
substructure of defined regions of polytene chro- 
mosomes (through a grant from the National Insti- 
tutes of Health) . 
Dr. Sedat is also Professor of Biochemistry and 
Biophysics at the University of California, San 
Francisco. 
Books and Chapters of Books 
Chen, H., Clyborne, W., Sedat, J.W., and Agard, 
D. 1992. PRIISM: an integrated system for display 
and analysis of 3-D microscope images. In 
Biomedical Image Processing and Three- 
Dimensional Microscopy (Acharya, R.S., Cogs- 
well, C.J., and Goldgof, D.B., Eds.). Bellingham, 
WA: International Society for Optical Engineer- 
ing, vol 1660, pp 784-790. 
Kam, Z , Chen, H., Sedat, J.W., and Agard, D. 
1992. Analysis of three-dimensional image data: 
display and feature tracking. In Electron Tomog- 
raphy: Three- Dimensional Imaging with the 
Transmission Electron Microscope (Frank, J., 
Ed.). New York: Plenum, pp 237-256. 
Article 
Paddy, M R., Agard, D.A., and Sedat, J.W. 1992. 
An extended view of nuclear lamin structure, 
function, and dynamics. Semin Cell Biol 3:255- 
266. 
CHEMICAL MECHANISMS IN CELLULAR REGULATION 
PaulB. Sigler, M.D., Ph.D., Investigator 
Dr. Sigler's laboratory continues to study the 
mechanisms of two cell regulatory processes: con- 
trolled gene expression and transmembrane signal- 
ing. The approach is to crystallize the macromole- 
cules in complexes that reveal the stereochemical 
basis of their function. The mechanisms inferred 
from these studies are tested by biochemical, physi- 
cochemical, and directed mutational studies. 
Whereas the short-range goal is to visualize the ste- 
reochemical mechanisms, the ultimate goal is to de- 
scribe the physical chemistry of these cellular pro- 
cesses in energetic and dynamic terms. 
Transcriptional Regulation: The Chemistry 
of Specific Protein-DNA Interactions 
Crystallographic studies have been undertaken on 
a range of prokaryotic and eukaryotic regulatory 
complexes, with a view to explaining in chemical 
terms how transcription regulators are targeted to 
their specific DNA segments. 
The trp repressor- operator complex. The unex- 
pected stereochemistry of the specific interface 
seen in the high-resolution crystallographic analysis 
of the trp repressor-operator complex provoked the 
assertion that an incorrect DNA target was cocrystal- 
lized. This contention has been refuted and shown 
to have arisen from experimental artifacts in gel 
shift analyses. 
Two mechanisms of sequence recognition that 
were given strong credence by this crystal structure 
have now been experimentally confirmed. The first 
was that the water molecules could mediate specific 
interactions. A non-operator sequence designed to 
provide a water-mediated interaction similar to that 
seen in the natural complex does, indeed, support 
specific high-affinity binding of the repressor. 
The second mechanism, indirect readout, posits 
that the target sequence can be more easily de- 
formed than other sequences to assume the nonca- 
nonical structure exhibited in the complex. This 
suggestion has been confirmed in a collaborative 
study with Dr. Zippora Shakked and Dov Rabino- 
vitch of the Weizmann Institute in Israel. The crystal 
structure of an uncomplexed DNAdecamer contain- 
STRUCTURAL BIOLOGY 485 
